[petsc-users] PCFIELDSPLIT on a MATNEST with MATSHELL off-diagonal blocks

[Le Chenadec, Vincent]

Dear all,

I'm trying to use PCFIELDSPLIT as a preconditioner on a block matrix A,

[A_{00}   A_{01}]
[A_{10}   A_{11}]

where the diagonal blocks (A_{00} and A_{11}) are diagonally dominant. I would like to use either one of the block preconditioners (not even based on the Schur complement, simply either one of the Jacobi/Gauss-Seidel ones).

The issue I'm running into concerns the formats I would like to use, so I'll start with that:

  1.  A_{00} and A_{11} are both in MATMPIAIJ format (created using DMCreateMatrix on 2D DMDAs of different sizes),
  2.  A_{10} and A_{01} are both in MATSHELL format (created using DMCreateShell). The reason why I used this format is that it seemed convenient, since the 2D DMDAs are of different sizes and I assumed I had to provide the MATOP_MULT operation only,
  3.  Finally the blocks are stored in MATNEST format (MatCreateNest).

With this choice, matrix-vector multiplications work fine (the DMDAs being packed in a DMComposite), which makes me think that so far the code is ok. Also, when I attempt to invert the matrix without preconditioner, I get the correct result.

Now here is where things do not work: if I try to use a preconditioner, I get the error pasted below (for a simple PCJACOBI)


[0]PETSC ERROR: No support for this operation for this object type

[0]PETSC ERROR: Mat type shell

[0]PETSC ERROR: #1 MatGetDiagonal() line 4306 in petsc/3.6.3_dbg/src/mat/interface/matrix.c

[0]PETSC ERROR: #2 PCSetUp_Jacobi() line 180 in petsc/3.6.3_dbg/src/ksp/pc/impls/jacobi/jacobi.c

[0]PETSC ERROR: #3 PCSetUp_Jacobi_NonSymmetric() line 261 in petsc/3.6.3_dbg/src/ksp/pc/impls/jacobi/jacobi.c

[0]PETSC ERROR: #4 PCApply_Jacobi() line 286 in petsc/3.6.3_dbg/src/ksp/pc/impls/jacobi/jacobi.c

[0]PETSC ERROR: #5 PCApply() line 483 in petsc/3.6.3_dbg/src/ksp/pc/interface/precon.c

[0]PETSC ERROR: #6 KSP_PCApply() line 242 in petsc/3.6.3_dbg/include/petsc/private/kspimpl.h

[0]PETSC ERROR: #7 KSPSolve_CG() line 139 in petsc/3.6.3_dbg/src/ksp/ksp/impls/cg/cg.c

[0]PETSC ERROR: #8 KSPSolve() line 604 in petsc/3.6.3_dbg/src/ksp/ksp/interface/itfunc.c

[0]PETSC ERROR: #9 PCApply_FieldSplit() line 988 in petsc/3.6.3_dbg/src/ksp/pc/impls/fieldsplit/fieldsplit.c

[0]PETSC ERROR: #10 PCApply() line 483 in petsc/3.6.3_dbg/src/ksp/pc/interface/precon.c

[0]PETSC ERROR: #11 KSP_PCApply() line 242 in petsc/3.6.3_dbg/include/petsc/private/kspimpl.h

[0]PETSC ERROR: #12 KSPSolve_CG() line 139 in petsc/3.6.3_dbg/src/ksp/ksp/impls/cg/cg.c

[0]PETSC ERROR: #13 KSPSolve() line 604 in petsc/3.6.3_dbg/src/ksp/ksp/interface/itfunc.c

I may not be calling the appropriate routines, here is what I'm basically doing:

  1.  KSP type set to KSPCG (A is symmetric), PC type to PCFIELDSPLIT,
  2.  I then use MatNestGetISs on A to supply the ISs to PCFieldSplitSetIS,
  3.  Finally I use PCFieldSplitGetSubKSP to use set the subKSP for each diagonal block.

As I mentioned, as long as the subKSPs don't have any preconditioning (PCNONE), KSPSolve does not complain, only when I try to using one (even PCJACOBI) do I get similar errors). That makes me suspect that I'm doing something wrong, but I could not find an example that was doing the same thing...

Any pointers would be much appreciated!

Thanks,
Vincent
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